Subversion 1.9 working copy compatibility

[Marc Strapetz]

From my experiments with Subversion 1.9 binaries and the listed changes 
in the release notes, Subversion 1.9 seems to be backwards compatible 
with Subversion 1.8 working copies. Is that correct? If so, it makes 
sense to update the "Upgrading the Working Copy" section:

https://subversion.apache.org/docs/release-notes/1.9.html#wc-upgrade
At least I was initially reluctant to try Subversion 1.9 on my 
real-world working copies to avoid troubles when switching back to 1.8.

-Marc